1. Field of the Invention
The present invention relates to a method for controlling handoff in mobile communications system, in which change of service area of a base station in response to increase of users is obtained and traffic channel gain of Code Division Multiple Access (CDMA) channels in dummy pilot method is appropriately changed to adjust service areas, so that successful handoff may be performed, reducing disconnection ratio of calls.
2. Description of the Conventional Art
FIG. 1 represents handoff boundary between base stations in a general mobile communications system.
In general, as shown in FIG. 1, cellular communications systems, personal communications systems, satellite communications systems and the like which are using the CDMA technology, have a boundary between coverage areas of two different base stations A and B, that is, a handoff area C exists between the base stations A and B.
In the handoff area, successful handoff has to be performed in order to maintain continuous communications (in order to prevent disconnection of a call) and prevent power loss.
FIG. 2 is a graph representing a handoff area between base stations in a general mobile communications system.
In the CDMA system, lists of base stations, which are expected as candidates for handoff, and handoff threshold values are transmitted to a mobile station. The mobile station detects not only pilot signal strength of a base station which is presently in service but also those of neighboring base stations which are the handoff candidates, and compares their pilot signal strengths with the threshold values.
In FIG. 2, the handoff area represents an area where the mobile station moves from when the pilot signal strength of the neighboring base station B exceeds a threshold value T_ADD to when a delay time T_TDROP lapses after that the pilot signal strength of the currently active base station A decreases below a threshold value T_DROP.
In the handoff area, efficient control for the handoff should be performed.
Looking into the conventional control method for the cellular mobile communications system using the CDMA technology, handoff failure usually happens when the mobile station moves from an overpopulated downtown area having many users to a suburb area having few users, due to the different base stations having the different number of CDMA channels.
In this case, in order to prevent the handoff failure, dummy pilot method is utilized, wherein there is no traffic channel and only overhead channels exist. That is, in the places, where different base stations having the different number of channels are adjacent to each other, dummy pilot is utilized to support hard handoff in order to save the hard handoff cost between the CDMA channels.
In other words, as shown in FIG. 1, the base station A has two channels FA0, FA1 and the base station B has a channel FA0, so that it is possible to use dummy pilot which has no traffic channel for the hard handoff and only the overhead channel.
However, increase in the number of users causes increase of the interruption and decrease of the service area of a base station. Therefore, the CDMA channel FA0 having traffic channels decreases a lot in its service area and the CDMA channel FA1 having no traffic channels has little change in its service area. Accordingly, there happens a difference in the service areas of the two base stations. At this time, if a user communicates in service area of the base station A using FA1 and moves to the service area of the base station B while communicating, the hard handoff happens, which is apt to be failed due to the difference of the service areas between the CDMA channels FA0 and FA1.
In other words, the conventional technique has disadvantages that the hard handoff is apt to be failed due to the difference of the service areas between the CDMA channel which uses the dummy pilot and the CDMA channel which does not use the dummy pilot, since the service area of a base station decreases, as the number of users increases.
Further, the conventional technique has disadvantages that the call connection may be failed even in case that the user tries to communicate in the handoff area D between the base stations A and B.
The present invention is derived to resolve the disadvantages of the conventional techniques and it is an object of the present invention to provide a method for controlling handoff in mobile communications system, in which hard handoff may be performed successfully and call disconnection may be prevented by obtaining change in the service areas of base stations according to the increase of the number of users and adjusting the service area by changing traffic channel gains of CDMA channels having the dummy pilot.
In order to achieve the above objects of the present invention, a method for controlling handoff in mobile communications systems, comprising the steps of a first step, in a base station, for making output changes from the base station for a number of users into database to store the database in an inner memory and detecting a number of channels which are in use for Code Division Multiple Access (CDMA) channels to store the number in the inner memory, a second step, in a mobile station, for measuring and storing pilot power strength of a base station, which is currently in service, and continuously detecting pilot power strength of neighboring base stations, so that a pilot strength measurement message is transmitted if a pilot strength, which is included in a neighbor list of pilot offset lists of the mobile station, exceeds a threshold value, a third step, in the base station, for analyzing the transmitted pilot strength measurement message to determine as to whether it is handoff procedure or not; and a fourth step, in the base station, for instructing hard handoff if handoff procedure is determined in the third step and adjusting traffic channel gain of a corresponding CDMA channel according to the number of users by using the change of output from the base station for the number of users, which is stored in the first step, to make service areas between the CDMA channels, so that the mobile station is assigned with forward power and code channels, wherein in the fourth step, the adjustment of the traffic channel gain of the traffic channels is performed in such a manner that traffic channel gain from the CDMA channels increases as the number of users increases.